Nightmares experienced by adults are quite spontaneous in the majority of instances however it's also possible for them to be a symptom of something else.
You may find that you experience nightmares with a greater frequency after a late night snack which will often result in an increased rate of metabolism and more activity in the brain.
Certain medications such as antidepressants or narcotics are also known to result in a greater frequency of nightmares during sleep.
It should be noted that withdrawal from a dependence on these same substances will also cause disturbed sleep and nightmares.
Psychological reasons can also be responsible such as if you suffer from panic attacks or a panic disorder, depression and anxiety.
One condition in particular is known to cause chronic and recurring nightmares - post traumatic stress disorder.
If you have relatives in the family which also suffer from nightmares then there's a distinct possibility that this has been passed on to you as well.
If the other areas of your life are normal then the occasional nightmare is perfectly harmless and should not have any adverse effect upon you.
If you do suffer from other disorders such as anxiety or depression then nightmares could increase your level of distress and have a greater impact upon your well-being.
If nightmares are significantly interrupting your sleep then take some action and see your doctor, sleep deprivation is serious and over time can result in a variety of medical conditions such as heart disease or obesity to name just a couple.